Question

In Go, how do you create the instance of an object from its type at run time? I suppose you would also need to get the actual type of the object first too?

I am trying to do lazy instantiation to save memory.

Answer 1

In order to do that you need reflect.

package main

import (
    "fmt"
    "reflect"
)

func main() {
    // one way is to have a value of the type you want already
    a := 1
    // reflect.New works kind of like the built-in function new
    // We'll get a reflected pointer to a new int value
    intPtr := reflect.New(reflect.TypeOf(a))
    // Just to prove it
    b := intPtr.Elem().Interface().(int)
    // Prints 0
    fmt.Println(b)

    // We can also use reflect.New without having a value of the type
    var nilInt *int
    intType := reflect.TypeOf(nilInt).Elem()
    intPtr2 := reflect.New(intType)
    // Same as above
    c := intPtr2.Elem().Interface().(int)
    // Prints 0 again
    fmt.Println(c)
}

You can do the same thing with a struct type instead of an int. Or anything else, really. Just be sure to know the distinction between new and make when it comes to map and slice types.